The present disclosure relates to a lighting device such as a light therapy device. The disclosure further relates to a method of masking an edge transition of transmitted light behind the circumference of an edge structure in a lighting device.
Light therapy consists of the exposure of a subject to a prescribed dosage of light. For example, light having a particular frequency may be used to treat circadian rhythm disorders. For the light therapy device to be effective, it typically needs a relatively high light output. To prevent blinding (or to improve eye comfort), the light can be distributed over a relatively large surface of the device, e.g. using a light guide plate. However, especially when operating at a high light output, any edges of components in the device, including an edge of the light guide, may cast an edge transition (shadow) on the light emitted from the device. Such shadows and sharp edge transitions may cause a user to experience ghost (after) images in his retina. Accordingly, to improve eye comfort it is desired to provide a gradient light source, i.e. masking a sharp transition between the bright display and a first area having a relatively low transparency.
To mask an edge transition, a gradient pattern at an exit surface of the lighting device can be used to provide a transition from light to dark at an edge of the display surface. For example, WO2010117114 describes a display apparatus including a display module, a front panel provided in front of the display module, and a pattern which is formed over the front panel. One portion of the pattern that is closer to an edge of the front panel allows less transmittance of light than other portions of the pattern. However, to provide a smooth edge transition for a very bright light source, the pattern needs to extend over a considerable distance, which may lead to deterioration of the total light output of the device.
Accordingly, it is desired to improve the masking of an edge transition of a light output window with minimal sacrifice to its light output efficiency.